Heretofore, the performance of capacitance based sensing or capacitive sensors have been limited. Depending on the physical structure, the capacitance between a target electrode and a sense electrode varies inversely proportional to the relative distance between them, inversely proportional to the relative distance between them squared, or some functional dependence between inverse and inverse quadratic. The maximum distance at which a capacitance sensor system can detect target conductors in the vicinity of its sensor area is dependent on the minimum capacitance the system can resolve. If the capacitance of the sensor electrode relative to its ambient environment, its reference capacitance, is large compared to the capacitance between the target electrode and the sensor electrode, the capacitance sensor system sensitivity is significantly degraded. The sensor electrode has significant reference capacitance alone. The size of the sensor electrode is dictated by the size of fingerprint artifacts, which is typically about the size of a 100 micrometer square. Being part of an integrated circuit whose vertical dimensions are small compared to 100 micrometers, the sensor electrode itself has significant capacitance to the substrate on which it physically or mechanically rests.
For use in measuring the positions of fingerprint artifacts, a sensor array composed of a rectangular grid of sensor electrodes was disclosed by Knapp in U.S. Pat. No. 5,325,442. Each sense electrode is connected through a passive switch to array wiring that is the length of the array. The array wire is connected to a charge sensing circuit to determine the capacitance. The capacitance sensitivity is degraded by the array wiring as the effective reference capacitance on each sensor electrode increased. Additionally, semiconductor switches are introduced into the sensor area where they may be damaged by mechanical contact with the target electrode, or may leak due to photocurrent in the sensor is operated in a high light level environment. Additional coatings may be applied to the sensor surface to reduce the sensor's susceptibility to damage, but at an increase in the sensor to target electrode distance.
In U.S. Pat. No. 6,049,620 Dickinson et al. disclose a technique to measure the capacitance at each sensor electrode using a low value current source and additional active circuitry. A signal proportional to the capacitance is switched onto the array wiring which no longer degrades the capacitance sensor system sensitivity. The reference capacitance value is dominated by the sensor electrode capacitance and the capacitance of the circuitry connected to the sensor electrode itself.
In U.S. Pat. No. 6,097,195 Ackland et al. disclose a method to reduce the sensor electrode capacitance by introducing a shield electrode between the sensor electrode and the physical support structure at ground potential. This reference capacitance cancellation technique is applied individually to each sensor electrode, resulting in some reduction in the reference capacitance and a proportional increase in the sensor capacitance sensitivity. The amplifier used in the feedback circuit was a source follower whose gain was significantly less than unity. This resulted in incomplete reference capacitance cancellation, but required no additional circuitry and little additional power.
Other capacitive sensor systems have been described which add circuitry to the sensor array. In U.S. Pat. No. 6,114,862 Tartagni et al. disclose a capacitance sensor with active circuitry and special electrode configurations designed to improve the capacitive sensor sensitivity. This increase in sensor complexity increases the risk of damage to the sensor from various sources and degradation form others. The cost of the sensor system as and integrated circuit and its risk of damage are proportional to the sensor size, which is usually over 1 cm square for the nearly rectangular sensor arrays.
Rather than measuring the static position of the fingerprint artifacts, in U.S. Pat. No. 6,317,508 Kramer et al. disclose a capacitive sensor with a rectangular aspect ratio over 10:1. This sensor can measure capacitances and locations of fingerprint positions as the finger is moved over the sensor surface. Such asymmetrical arrays offer a cost advantage proportional to their integrated circuit size reduction. The asymmetry also reduces the effects of the array wiring in sensors disclosed by Knapp for wires along the narrow sensor direction.
While these known designs and methods present opportunities to improve the performance and reduce the cost of capacitive sensors, problems and limitations still remain, at least some of which are solved by the invention disclosed herein.